A thermal printer has recently attracted attention, and a dye thermal transfer printer capable of printing a clear full color image has attracted particular attention. In a dye thermal transfer printer, a dye layer containing a dye of an ink ribbon is superposed on an image receiving layer (hereinafter sometimes simply referred to as a “receiving layer”) containing a dye-dyeable resin of a receiving sheet, and the dye of the dye layer in a required portion is transferred at a predetermined concentration to the receiving layer by the effect of heat supplied from a thermal head or the like, whereby an image is formed. In the ink ribbon, three-color dye layer regions of yellow magenta and cyan or four-color dye layer regions of these three colors and black are sequentially provided. A full color image is obtained by repeatedly transferring respective color dyes, in sequence, to a receiving sheet. In the case of such a dye thermal transfer-system printer, the receiving sheet is generally fed in the flat sheet state.
With the progress of a digital image processing technique using a computer, the image quality or the like of an image recorded by the dye thermal transfer system is remarkably enhanced and the market for this system is expanded. Furthermore, the technique of controlling the temperature of a thermal head is improved and, along therewith, the demand for a high-speed high-sensitivity printing system is increasing. Therefore, how efficiently the heat value of a heating device such as thermal head is utilized for the image formation is an important problem to be solved. Also, low cost, simplified structure and the like of a printer are required, and this brings about a technical problem such as reduction in the printing pressure by a thermal head and prolongation of the head life. At present, a printer capable of printing one A6 size sheet within 30 seconds is available on the market, and it is expected that the demand for printing at a higher speed will increase in the future.
For efficiently forming a high-quality and high-density image, a receiving sheet comprising a support having provided thereon a receiving layer mainly comprising a dye-dyeable resin is generally used, but when a normal film is used as a substrate for the support, despite excellent smoothness, the heat from a thermal head escapes to the substrate to cause insufficient recording sensitivity or, as a film is lacking in the satisfactory cushioning property, close contact between the ink ribbon and the receiving sheet is unsatisfactory and, as a result, density unevenness or the like is generated.
In order to solve such problems, for example, a support obtained by laminating a foamed film with a core material layer such as paper sheets (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 61-197282 (page 1)), and a support obtained by laminating a biaxially stretched film (synthetic paper) mainly comprising a thermoplastic resin such as polyolefin resin and containing a void structure, with a core material layer such as paper sheets (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 62-198497 (page 1)) have been proposed as the support. The receiving sheet using such a support is excellent in the heat insulating property and smoothness but disadvantageously fails in having a paper-like texture or is expensive.
Furthermore, when a paper sheet is used as the support of the receiving sheet, similarly to the film, the recording sensitivity is insufficient and, although the cushioning property is somewhat higher than that of a film, the close contact between the ink ribbon and the receiving layer becomes non-uniform due to the uneven fiber density of the paper and the print tends to have irregular shading. Therefore, for the improvement of transfer density or the like, a receiving sheet where an intermediate layer containing hollow particles is provided between the paper support and the receiving layer has been disclosed (see, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 63-87286 (pages 1 and 2) and 1-27996 (pages 1 to 3)). In this receiving sheet, the sensitivity is improved by the effect of enhancing the heat insulating property or cushioning property of the hollow particle-containing layer, but the hollow particles are liable to produce an irregularity on the receiving sheet surface.
With respect to the improvement of irregularity on the receiving sheet surface, a receiving sheet having, for example, a specific surface roughness or glossiness by specifying the average particle diameter or hollow percentage of a hollow particle used in the intermediate layer has been proposed (see, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 9-99651 (pages 1 to 5) and 2001-39043 (pages 2 and 3)). Also, a receiving sheet comprising a substrate sheet having formed thereon a resin layer containing an air bubble layer and a receiving layer, where a smoothing treatment is applied to the air bubble layer and/or the receiving layer, has been proposed (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-210968 (pages 2 to 4)).
However, a satisfactory correlation is not necessarily present between the surface roughness value of the receiving layer as measured by a conventional measuring method and the quality of an image actually obtained by a dye thermal transfer printer. Particularly, a good image quality can hardly be obtained in the printing by a current high-speed printer at a low printing pressure. Furthermore, when the hollow percentage by volume of the hollow particle is elevated, the receiving sheet surface is readily scratched. That is, there is a problem that generation of a scratch is liable to occur resulting from hitting with a nail, a pen point or the like against the sheet surface on handling the printed matter and the commercial value is seriously decreased.
Also, to improve the smoothness on the receiving sheet surface, a receiving sheet having a porous layer comprising a plurality of layers, where a laminate layer formed by co-extrusion of a thermoplastic resin is provided on the porous layer, has been proposed (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-272259 (page 2)). However, as a resin solution containing an air bubble formed by stirring or a thermally expansible resin of forming an excessive hollow is used for the porous layer on the core material layer side, sufficient smoothness can hardly be obtained or the formation of the laminate layer may cause, for example, deformation and collapse of the porous structure of the porous layer due to heat and the effect of enhancing the sensitivity and image quality is not necessarily at a satisfactory level. Furthermore, a receiving sheet having a receiving layer into which two or more kinds of fine hollow particles differing in particle diameter are incorporated has been proposed (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 11-291647 (page 2)), but the hollow particles present in the receiving layer cannot satisfactorily provide the effect of enhancing the heat insulating property or cushioning property and adversely affect the dyeability or the like of the receiving layer and, as a result, the image tends to have insufficient uniformity.
In addition, the partition wall of the hollow particle used in such a receiving sheet having an intermediate layer is formed of a polymer material having a low glass transition temperature and, therefore, the hollow particle is generally poor in the heat resistance and may be thermally deform and collapse due to heat at the production of the receiving sheet or heat from a thermal head at the printing, making it difficult to control the printing density, or the printed area may be dented due to heat to impair the outer appearance. Accordingly, a sufficiently high image quality may be unobtainable in the printing by a current high-speed printer.
Also, a receiving sheet comprising a paper substrate having sequentially stacked thereon a hollow particle-containing layer and a dye receiving layer, where the cushion deformation percentage of the entire receiving sheet is from 10 to 30%, has been proposed (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-200851 (pages 2 to 5)). Although the construction material for the partition wall of the hollow particle is selected from the standpoint of enhancing the solvent resistance, as the heat resistance is not taken into account at all, a need for improvement still exists.